Post-Eruption Arcades and Interplanetary Coronal Mass Ejections

16 Pages·2013·0.79 MB·English

Post-Eruption Arcades and Interplanetary
Coronal Mass Ejections
S. Yashiro1,2, N. Gopalswamy2, P. Mäkelä1,2, and S. Akiyama1,2
1: The Catholic University of America, Washington, DC 20064, USA
2: NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
[email protected]; [email protected]; [email protected];
[email protected]

Abstract We compare the temporal and spatial properties of post-eruption arcades (PEAs)
associated with coronal mass ejections (CMEs) at the Sun that end up as magnetic cloud (MC) and
non-MC events in the solar wind. We investigate the length, width, area, tilt angle, and formation
time of the PEAs associated with 22 MC and 29 non-MC events and we find no difference
between the two populations. According to current ideas on the relation between flares and CMEs,
the PEA is formed together with the CME flux-rope structure by magnetic reconnection. Our
results indicate that at the Sun flux ropes form during CMEs in association with both MC and non-
MC events; however, for non-MC events the flux-rope structure is not observed in the
interplanetary space because of the geometry of the observation, i.e. the location of the spacecraft
when the structure passes through it.
KEYWORDS: Flares; Coronal Mass Ejections; Post-eruption arcades; Flux rope;
Magnetic cloud

1. Introduction
Two-ribbon flares are characterized by a pair of bright ribbons observed in Hα
and ultraviolet (UV) images. The ribbons are located on either side of a magnetic
polarity inversion line and they separate from each other as the flare progresses.
Two-ribbon flares are often associated with filament eruptions and coronal mass
ejections (CMEs). After the launch of the filament, long-lived arcades are formed
connecting the two ribbons across the polarity inversion line. The emerged
assembly of arches is called a post-eruption arcade (PEA). The PEAs are observed
in multiple wavelengths and are known also as long duration (or decay) events
(LDEs; Pallavicini, Serio, and Vaiana, 1977) in the X-ray observations. The
erupting filament becomes the core of the associated CME (Webb and
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Hundhausen, 1987; Gopalswamy et al., 2003), thus PEAs are considered as
surface signatures of CMEs (Tripathi, Bothmer, and Cremades, 2004).
The CSHKP magnetic reconnection model is widely accepted as a model of
two-ribbon flares and CME onsets. The acronym “CSHKP” stands authors of this
model, Carmichael (1964), Sturrock (1966), Hirayama (1974), Kopp and
Pneuman (1976). This model suggests that during a filament eruption, a current
sheet is formed and magnetic reconnection occurs below the erupting filament.
The reconnected field lines arching downward to the solar surface form the PEA,
while the upward reconnected field lines envelop the erupting filament and form
the flux-rope structure (see e.g., Longcope and Magara 2004; Longcope and
Beveridge, 2007). This model explains well the characteristics of two-ribbon
flares: the separation of flare ribbons, the development of the PEAs, and the
association with filament eruptions and CMEs (Bruzek 1964; Sheeley et al. 1975;
Kahler 1977; Munro et al. 1979; Harra-Murnion et al. 1998; Yurchyshyn 2008).
Hα flare ribbons form the feet of the PEAs at the chromospheric level. At the
coronal level, the PEAs are observed as a collection of loops in X-rays, EUV, and
even microwaves (e.g., Hanaoka et al., 1994; McAllister et al., 1996;
Gopalswamy et al., 1999; 2003; Tripathi, Bothmer, and Cremades, 2004). The
model is also supported by the relationship between the reconnected magnetic
flux during flares and the magnetic flux of MCs observed near Earth, as reported
by Qiu et al. (2007).
There is a general consensus that large PEAs appearing near the central
meridian of the Sun are good indicators of geoeffective CMEs. The Bastille Day
flare on 2000 July 14, with a bright EUV arcade (see Figure 1a, see also
Aschwanden and Alexander 2001), caused an intense geomagnetic storm with a
minimum Dst of –301 nT. During an earlier event on 1994 April 14, when
coronographic observations of CMEs were more limited, the Soft X-ray Telescope
on the Yohkoh satellite observed a giant PEA that extended 150° in longitude. An
Earth-directed CME was expected and an alert of a geomagnetic storm was issued
(McAllister et al. 1996). We should note that the X-ray intensity of the event was
very low; the X-ray Sensor on the Geostationary Operational Environmental
Satellites (GOES) did not detect any significant X-ray enhancement. This case
showed that a large-scale PEA is a good indicator of CMEs arriving at Earth and
causing geomagnetic storms. The Extreme-ultraviolet Imaging Telescope (EIT) on
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(a) (b) (c)
C
M
(d) (e) (f)
(g) (h) (i)
C
M
-
n
(j) (k) (l)
o
N

Figure 1 – PEAs observed by EIT (1st and 3rd rows) and the associated
CMEs observed by LASCO (2nd and 4th rows). The top 2 rows show the
solar sources of MC events, while the bottom 2 rows are for non-MC events.
board the Solar and Heliospheric Observatory (SOHO) mission started observing
PEAs routinely since 1996 and helped to identify the solar sources of CMEs
observed by the Large Angle and Spectrometric Coronagraph (LASCO;
Brueckner et al., 1995).
Two Coordinated Data Analysis workshops (CDAWs) were held in 2010 and
2011 to address the question: Do all CMEs have a flux-rope structure? This
question derives from the fact that only a fraction of the interplanetary CMEs
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(ICMEs; i.e. the plasma and magnetic field structure in interplanetary space
corresponding to CMEs from the Sun) are observed as flux ropes or magnetic
clouds at 1 AU (MCs; Burlaga et al. 1981). ICMEs without an observable flux-
rope structure are called “ejecta” or non-MCs.
It has been suggested that non-MCs may have a flux-rope structure but that it
may not be observed in single-point measurements at 1 AU if the observing
spacecraft passes through the periphery of the ICME (Gopalswamy, 2006). If this
were the case, one does not expect any difference between PEAs associated with
MCs and non-MCs at the Sun. Such a view has been supported by the fact that a
flux rope can be fit to all white-light CMEs near the Sun, irrespective of their 1-
AU manifestation as MCs or non-MCs (Xie, Gopalswamy, and St. Cyr, 2013). As
CMEs propagate away from the Sun, the trajectories of those CMEs associated
with non-MCs become non radial (Xie et al., 2013; Kim et al., 2013) due to
deflection by nearby coronal holes (Mäkelä et al., 2013). Other possibilities are
CME-CME interactions leading to complex ejecta (Burlaga, Plunkett, and St. Cyr,
2002) or inherent non flux-rope structure at the Sun (Gosling, 1990). Gopalswamy
et al. (2013) have shown that both MCs and non-MCs have charge-state
enhancements at 1 AU, which is an indication of flare reconnection and flux-rope
formation at the Sun, thus ruling out the non flux-rope structure suggested by
Gosling (1990).
If CMEs resulting in non-MCs do not have a flux-rope structure and if the flux-
rope structure of the MC-associated CMEs is formed by magnetic reconnection,
as suggested by the CSHKP model, we should be able to find differences between
the MC-associated and the non-MC-associated PEAs. The aim of this paper is to
examine this possibility by investigating the differences of the PEAs observed in
EUV between the two populations.

2. Data Set and Analysis
The data set used in this study consists of the CDAW events selected from the list
of shock-driving ICMEs listed in Gopalswamy et al. (2010). From the more than
200 events listed in Gopalswamy et al. (2010), 59 events originating from the disk
center (±15° from the central meridian) were selected to form the CDAW list. If
one assumes that all CMEs have a flux-rope structure then many of the IP drivers
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at 1 AU should be MCs because solar eruptions launched from disk center are
expected to hit Earth head-on. However, only 24 events were MCs and the
remaining 35 events were classified as ejecta or non-MCs. Comparisons of these
two populations were the suggested agenda of the workshops.
A careful analysis of the selected 59 events was carried out by the meeting
participants during and after workshops. It turned out that the revised solar source
of 5 events (No. 6, 11, 12, 22, and 55) did not meet the original location criteria
(for details see Gopalswamy et al., 2013). Therefore, the number of events in the
CDAW list was reduced to 54. In this study we have excluded two events (No. 1
and 58) because of EIT data gaps. We have also excluded one event (No. 40)
because no flare brightening was identified after the filament eruption.
Table 1 summarizes the properties of the 51 PEAs analyzed in our study. Note
that the event numbers (column 1) are not sequential because we use the event
numbers of the original 59 events. The ICME type is given in the column 2: MC
for magnetic clouds and EJ (ejecta) for non-MC. General information on the solar
sources are given in columns 3 – 7 (column 3: the flare date in yyyy/mm/dd
format; column 4: the flare start time as hh:mm in UT; column 5: the flare
location in the heliographic coordinates; column 6: the soft X-ray flare class;
column 7: the CME speed in km s-1). Columns 8 – 12 give the PEA information:
the length in Mm, the width in Mm, the size in Mm, the tilt angle in degree, and
the PEA developing time in hours (see Section 3 for their definitions).
In 31 events, out of the 51 analyzed, we could see well-developed PEAs.
Figure 1a shows a PEA observed during the so-called Bastille Day event on 2000
July 14 (No. 19). The X-ray class of the associated flare was X5.7 and the flare
duration was 40 minutes. This event was associated with an Earth-directed CME
which arrived at Earth 38 hours later as an MC. This event provides a good
example of a bright, large PEA associated with an MC. Figure 1g shows another
bright PEA but this one was associated with a non-MC (No. 47). The flare was an
LDE flare (duration > 4 hours) with peak X-ray class C5.5. The flare occurred at
S13W09 and was associated with a halo CME (see Figure 1j). Two days after the
corresponding ICME was observed by the Advanced Composition Explorer
(ACE). The ICME was classified as non-MC since no clear rotation of the
magnetic field was observed.
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In 10 events, we could identify long-lived non-arcade loops (see Figure 1b; No.
54) or short-lived arcades (see Figure 1h; No. 28). These events differ a little bit
from the typical appearance of PEAs (long-lived arcades). In the remaining10
events, we could see short-lived non-arcade flaring loop. Figure 1c shows an
example of an M8.0 flare at 02:43 UT on 2000 July 25 (No. 21). The flare was
compact (< 45 Mm) and impulsive (11 min). Usually, such compact impulsive
flares are likely to be confined (Yashiro et al., 2006), but this flare was associated
with an EIT dimming and a faint halo CME (Figure 1f). The halo CME appeared
in LASCO/C2 at 03:30 UT with an apparent speed of 528 km s-1 and, 3 days later,
the associated ICME was observed as an MC. One could not necessarily classify
these as PEAs but we have included them in the analysis because, according to the
CSHKP flare-CME model, bright flare loops and PEAs are not physically distinct.
The solar source of the non-MC event on 2000 October 5 (No. 25) was a halo
CME that appeared in the LASCO/C2 field of view at 03:50 UT on October 3
(Figure 1l). The main body of the CME appeared in the south-east quadrant and
the faint envelope surrounding it expanded to cover the occulting disk of the
LASCO/C2 coronagraph. The associated flare was the C4.1 flare at S09E07 at
02:48 UT on the same day. The flare was impulsive, so the EIT image observed
27 min after the flare peak (Figure 1i) does not show an arcade. These two
examples show that occasionally poor arcade signatures are observed both in MCs
and non-MCs.

3. Results
In order to investigate the spatial properties of the PEAs, we measured their
lengths, widths, and areas. Since PEAs grow larger in time, we determined their
properties when the size reached its maximum. We visually determined the
footpoints of the PEAs in EUV images and measured the length, which
corresponds to the two flaring ribbons in Hα images. The red lines of Figure 1
show the locations of the footpoints of each PEA. We define the length of the
PEAs as the average length of the two ribbons.
Figure 2a shows the distribution of the lengths of PEAs associated with MCs.
The lengths are distributed widely from 29 Mm to 313 Mm with an average of
120 Mm. Figure 2b shows the same as 2a but for non-MC events. Similarly, the
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8 20 12
(a) 22 MCs (c) 22 MCs (e) 22 MCs
s Ave 118 Mm s Ave 72 Mm s 10 Ave 84 Mm
nt 6 nt 15 nt
ve ve ve 8
E E E
of 4 of 10 of 6
r r r
e e e
b b b 4
m m m
u 2 u 5 u
N N N 2
0 0 0
0 1 00 2 00 3 00 0 100 200 0 100 200
Length [Mm] Width [Mm] Size (A1/2) [Mm]
8 15 10
(b) 29 non-MCs (d) 29 non-MCs (f) 29 non-MCs
s Ave 109 Mm s Ave 86 Mm s 8 Ave 90 Mm
nt 6 nt nt
ve ve 10 ve
E E E 6
of 4 of of
er er er 4
b b 5 b
m m m
Nu 2 Nu Nu 2
0 0 0
0 1 00 2 00 3 00 0 100 200 0 100 200
Length [Mm] Width [Mm] Size (A1/2) [Mm]

Figure 2 – Distributions of lengths (left), widths (center), and sizes (A1/2; right)
for MC (top) and non-MC (bottom) events. The averages of each distribution
are shown in the plot.
lengths of non-MC associated PEAs are distributed widely from 9 Mm to 215
Mm. The average length is slightly shorter but the difference in average length
between the MC and non-MC events is not significant. Tripathi, Bothmer, and
Cremades (2004) examined 236 PEAs using SOHO/EIT and reported that the
heliographic length of the PEAs ranged from 2° to 40°, which correspond to 24
Mm to 486 Mm. The lengths of PEAs associated with MCs are distributed within
the range of Tripathi et al., but for the non-MCs cases they are not. The minimum
length of 9 Mm is out of the range because our data sets include compact flares
without the arcade structure (see Figure 1i). We include them because flare loops
and PEAs are not physically distinct according to the CSHKP model.
The middle panels compare the width distributions of the PEAs of MC-
associated and non-MC-associated events. The width of each PEA is defined as an
average distance between footpoints at the two ends. The average widths are
similar in both populations. The width distribution of the MC-associated PEAs is
narrower than that of the non-MC events.
The right-hand panels of Figure 2 compare the PEA size for the MC-events and
non-MC events. We define the PEA sizes as square roots of the areas (A1/2)
7
6 12
(a) 22 MCs (c) 22 MCs
s s
nt Ave -2 ° nt 10 Ave 3.9 h
e e
v 4 v 8
E E

of of 6

r r
e e
b 2 b 4
m m
u u 2
N N
0 0
- 90 -45 0 45 90 0 5 10
Tilt [deg] T [hour]
6 10
(b) 29 non-MCs (d) 29 non-MCs
s s
nt Ave -9 ° nt 8 Ave 3.5 h
e e
v 4 v
E E 6

f f
o o

r r 4
e e
b 2 b
m m
2
u u
N N
0 0
- 90 -45 0 45 90 0 5 10
Tilt [deg] T [hour]

Figure 3 – Distributions of PEA tilt angles (left) and PEA developing times
(right) for MC (top) and non-MC events (bottom).
between the two ribbons. We found that the PEA size distributions are very
similar in both populations. The average PEA size is 85 Mm for the MC events
and 90 Mm for the non-MC events.
We define the tilt angle of the PEAs as the angle between the PEA axis and the
East-West line, measured counterclockwise in degrees. A horizontal PEA has a
tilt angle within ±10°. For example, the PEA of the Bastille Day event (Figure 1a)
has a tilt angle of –9°. A vertical PEA (e.g. Figures 1b, 1c, and 1h) a tilt angle
larger than 80° or lower than –80°. With this definition, tilt angles +90° and –90°
are identical. A PEA axis is determined as the straight line between the two mid
points of the ribbons at its ends. We used the same definition even for curved
PEAs. The direction of the flux-rope axis is not considered here because it is not a
property of the PEA itself.
Figure 3a shows the distribution of the tilt angle of the MC-associated PEAs.
The tilt angles are widely distributed from –90° to 90°. No clear trend is found in
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our sample. Figure 3b is the same as Figure 3a but for non-MC events. We could
not find any preferable tilt angle or any difference between the two distributions.
One could expect more vertical PEAs than horizontal ones because ordinary
sunspots are aligned horizontally, thus, the neutral line between them orients
vertically. However, active regions producing many flares and CMEs are highly
sheared and complex, thus, such simple consideration could not be valid.
In order to investigate the temporal properties of the PEAs, we use the time
between the flare start and the PEA peak, defined as the time when a PEA is fully
developed. We refer to this as the PEA developing time. PEA peak times are
visually determined, so this is a somewhat subjective parameter. For the same
event, different observers would select a different EIT frame as the PEA peak
time. Our internal person-to-person check indicated that the errors between
different observers are typically within 2 EIT frames or 24 minutes.
The right-hand panels of Figure 3 show the distribution of the PEA developing
time for MC-associated (top) and non-MC-associated (bottom) events. As we saw
in the other parameters, both distributions are very similar. The PEA developing
time ranges from 24 minutes to 15 hours with an average of 3.9 hours for MC-
events and 3.5 hours for non-MC-events, respectively. The average developing
time is approximately half of the average lifetime (6.8 hours; Tripathi, Bothmer,
and Cremades, 2004).
Since PEAs are the essential ingredients of solar flares, one might expect a
relation between PEA and CME properties: the larger PEAs are more likely to be
associated with faster CMEs. Since all our events originated the disk center, the
observed speed represents the expansion speed which is correlated with the radial
speed (Gopalswamy et al., 2009, 2012). Figure 4a plots the PEA size (A1/2)
against the CME speed, but we cannot see any clear relation between them. The
correlation coefficient is 0.08 for both MC and non-MC events and 0.09 for all
data points.
It is known that the CME kinetic energy is proportional to the X-ray peak
intensity, but the correlation coefficient is only 0.54 (Hundhausen 1999, Yashiro
and Gopalswamy 2009). Figure 4b shows the correlation between the X-ray peak
intensity and the CME speed. The correlation coefficient for all data points
increases to 0.71.
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(a) :: 2214 MNMCsCs r= 0.09 (b) :: 1110 MNMCsC s(A (1A/21>/2 >8 .85. 5M Mmm) )
: 10 MCs (A1/2≤ 8.5 Mm)
: 14 NMCs (A1/2≤ 8.5 Mm)
s] s]
m/ m/
k k
d [ d [
e 1000 e 1000
e e
p p
S S
E E
M M
C C
r=0.71
r=0.85
r=0.62
100 100
4 6 8 10 12 14 10-7 10-6 10-5 10-4 10-3 10-2
PEA Size (A1/2) [Mm] X-ray Intensity [W/m2]

Figure 4 – (a) Plot of the PEA sizes (A1/2) and the CME speeds. Circles and crosses
denote the MC-associated and non-MC-associated events, respectively. (b) Plot of the
peak X-ray intensity and the CME speed. Six events lacking an observed X-ray flare
are excluded from the plot. Events with large (A1/2 > 8.5 Mm) and small sizes (A1/2 ≤
8.5 Mm) are shown in pink and green, respectively.

We divided our sample into 4 groups based on the ICME structure, i.e. MC
(circle) or non-MC (crosses) event, and on the PEA area, i.e. large (A1/2>8.5 Mm)
or small (A1/2 ≤ 8.5 Mm) PEA. The circles and crosses are highly overlapped so
we cannot see any difference between the MC and non-MC events. On the other
hand we can see a difference between the large (pink) and small (green) PEAs.
For a given flare intensity, the speeds of CMEs associated with larger PEAs are
higher on average. The correlation coefficient is 0.85 for large and 0.62 for small
PEA events.

4. Summary and Conclusions
Historic events, e.g. the Bastille Day Event on 2000 July 14 and the giant X-ray
arcade formation on 1994 April 14, have convinced us that energetic CMEs are
likely to be associated with a post-flare arcade formation. This idea is supported
by the CSHKP model (e.g. Longcope and Magara 2004, Longcope and Beveridge
2007), i.e. the flux-rope structure of a CME is formed by magnetic reconnection
which is also responsible for the formation of the PEA. If the non-MC associated
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